1. Field of the Invention
The present invention relates to a process for the production of polyethylene for blow molding.
2. Description of the Prior Art
Polyethylene is well known for use in the manufacture of blow molded articles, for example bottles. It is known in the art that polyethylene resin produced for the manufacture of blow molded articles must achieve a balance of (a) to physical properties of the resin so that the resultant blow molded article has the required physical characteristics and (b) processing properties of the resin so that the polyethylene melt may readily be processed into the blow molded article. In order to achieve good processability of the polyethylene resins, it is desired that the flow properties and the shear response of the polyethylene are improved by broadening the molecular weight distribution of the polyethylene. Moreover, the physical properties of the solid resin when employed for blow molding bottles require the resin to have a high density and a high environmental stress cracking resistance (ESCR).
As a general rule, a polyethylene having a higher density tends to have a higher degree of stiffness, thereby making it more suitable for blow molding into bottles. A higher stiffness in the known polyethylene increases bottle strength and enables thinner walls to be employed. However, in general, the environment stress cracking resistance of polyethylene has an inverse relationship with stiffness. In other words, as the stiffness of polyethylene is increased, the environment stress cracking resistance decreases, and vice versa. This inverse relationship is known in the art as the ESCR-rigidity balance. It is required, for any given bottle grade polyethylene, to achieve a compromise between the environmental stress cracking resistance of the polyethylene and the rigidity of the polyethylene employed in the blown bottle.
A number of different catalyst systems have been disclosed for the manufacture of polyethylene, in particular high density polyethylene (HDPE) suitable for blow molding. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced. It is known to employ a chromium-based catalyst (i.e. a catalyst known in the art as a "Phillips catalyst)". Such a chromium-based catalyst enables the production of polyethylene having desirable physical and rheological properties.
It is known in the art to use chromium-based catalysts to polymerize HDPE and in particular to produce high density polyethylene having high resistance to environmental stress cracking. For example, EP-A-0291824, EP-A-0591968 and U.S. Pat. No. 5,310,834 each disclose mixed catalyst compositions, incorporating chromium-based catalysts, for the polymerization of polyethylene. Each of those prior proposals suffers from the disadvantage that mixed catalysts are required which can increase the complexity and cost of the process.
U.S. Pat. No. 4,049,896 discloses an olefin polymerization catalyst and process in which the catalyst system comprises a chromium-based catalyst with aluminum and triethyl borane (TEB). The aluminum is typically present in an amount of 3.7 wt % based on the silica of the support for the catalyst and TEB is present in an amount to yield a 2.9 B/Cr atomic ratio. It is stated that the polymers have improved flow properties and shear response. However, there is no disclosure of the manufacture of high density polyethylene and the specification does not address the problem of achieving high ESCR in conjunction with high density.